1. Field of the Invention
The present invention relates generally to magnetic particle inspection (MPI) procedures for finding metallurgical flaws in steam turbine components and, more specifically, to a field strength indicator for testing magnetic field strength and direction prior to conducting an MPI.
2. Description of the Related Art
An MPI procedure involves magnetizing a part to be tested and then spraying the part with small magnetic particles. The basis behind performing MPI procedures is to have a magnetic field of sufficient strength oriented in the proper direction to find flaws. MPI works because the magnetic field (or flux) in the test part is disrupted at surface flaws and discontinuities. This disruption in the field results in flux leakage out of the surface of the part across the flaw or discontinuity trapping very small magnetic particles that are sprayed on the surface. Thus, MPI requires that the part being inspected be magnetic and that it be magnetized to a level that allows sufficient flux leakage around real flaws and discontinuities but not around false indications. The accepted terminology for this is field strength. Field strength must be sufficient to fully magnetize the part, in order to allow for leakage around flaws, without magnetically saturating any unflawed areas.
Field strength is critical to MPI in that if it is too weak indications will not be shown and if it is too strong, false indications will result. Field direction is also important in that it needs to be oriented as close as possible to perpendicular to the direction of the flaws. For these reasons, prior to MPI inspections, the magnetic field strength and direction are checked with a field strength indicator.
The field strength indicator is an important part of an MPI procedure in that it must attempt to simulate expected flaws in several different directions. This is necessary so that field strength and direction can be evaluated at the same time. Typical MPI field strength indicators are made by joining several "pie-shaped" pieces of carbon steel, usually eight, into a circle or octagon. A thin piece of non-magnetic material, usually copper about 0.010 inches thick, is then joined to one side of the circle. All joining is performed by brazing.
FIGS. 1 and 2 represent a typical pie-type field strength indicator as described above. FIG. 1 illustrates a field strength indicator 10 having eight carbon steel pie sections 12 which are brazed together to form artificial flaws 14 at the joints therebetween. The assembled pie sections 12 have a maximum diameter of about 1 inch and a thickness of about 1/8 of an inch. A pair of non-ferrous trunions 16 and 18 are brazed or otherwise mechanically attached at diametrically opposite points for attaching a non-ferrous handle 20 of any suitable length. As shown in FIG. 2, a copper plate 21, which is about 0.01 inches thick, is connected to one surface of the pie sections.
One of the drawbacks to the pie-type field strength indicator is that the joints between the pieces of carbon steel are relatively wide, being on the order of 0.010 to 0.030 inches. These wide artificial flaws are easy to magnetize and show; however, they are not representative of real flaws. This is because field strengths sufficient to show the pie indications are not sufficient to show real service-induced flaws and cracks. Because of the problems associated with the standard pie field strength indicators, there is a need to develop a field strength indicator that more closely represents real flaws.
Another drawback to the pie indicator, as well as other commercial indicators, is the material from which they are made. They are generally made from carbon steel which has significantly different magnetic properties than many of the materials, such as alloy steels, used for steam turbine components. Carbon steel has a higher permeability which allows it to accept more flux than other materials. As a result, it is possible to show indications in carbon steel field indicators at field strength that would not show indications in alloy steels.